A branch line coupler is the simplest type of quadrature coupler, since the circuitry is entirely planar. In a typical branch line coupler, two parallel transmission lines are connected together with two other parallel transmission lines. The transmission lines are λ/4 long and have two distinct characteristic impedances. A signal entering the port 1 is split into two quadrature signals on the ports 2 and 3, with the remaining port 4 fully isolated from the input port at the center frequency.
Branch line couplers usually do not have such a wide bandwidth and are constrained by their designed frequencies. As to the latter point, a branch line coupler has a single frequency, which can be used with a single device. To accommodate different frequency devices, it is necessary to use a differently designed branch line coupler. Accordingly, large chip area needs to be used to accommodate different operating frequencies.